The present invention relates to a molten carbonate fuel cell and, more particularly, to a molten carbonate fuel cell wherein an anode and a cathode are formed in contact with an electrolyte layer containing a carbonate and are porous layers, and the supply of reactive gas and the discharge of reaction products are performed through small pores in these electrodes.
When a gas which can be easily oxidized, such as hydrogen gas, and a gas which has a strong oxidizing power, such as oxygen, are reacted through an electrochemical reaction process, DC power can be generated. A fuel cell realizes such a reaction process.
Fuel cells are roughly classified based on the type of electrolyte used in fuel cells of phosphoric acid, molten carbonate, and solid electrolyte type. Of these types, molten carbonate fuel cells are operated at a temperature around 650.degree. C. Molten carbonate fuel cells do not require the use of a noble metal as catalyst, allow the use of carbon monoxides in place of hydrogen as fuel, have a relatively high voltage generated per unit cell, and allow utilization of high-temperature waste energy.
Such a molten carbonate fuel cell generally consists of a plurality of stacked unit cells. Output voltage from each unit cell is at best about 1 V. Thus, in order to obtain a practical cell, a plurality of such unit cells must be electrically connected in a series.
Each unit cell of such a molten carbonate fuel cell generally comprises an electrolyte layer in a plate-like form containing: a carbonate, melted at operating temperature; a cathode and an anode, which are formed in tight contact to sandwich the electrolyte layer and which comprise plates of porous material; an element for supplying fuel gas to the interface between the cathode and the electrolyte layer through pores in the cathode; and another element for supplying an oxidant gas to the interface between the anode and the electrolyte layer through pores in the anode. The elements for supplying the fuel and oxidant gases comprise a plurality of gas guide grooves, defined between those surfaces of conductive interconnectors inserted between each adjacent unit cells and which contact the unit cells when such unit cells are stacked. Gas guide grooves for guiding the fuel gas and those for guiding the oxidant gas are formed perpendicularly to each other, so as to allow each connection access to sources of the respective gases. The fuel gas guided by such gas guide grooves is diffused in the cathode in its direction of thickness toward the electrolyte layer. On the other hand, the oxidant gas guided through the corresponding gas guide grooves is diffused through the anode in its direction of thickness toward the electrolyte layer.
In a molten carbonate fuel cell of the type described above, the output characteristics are largely dependent on the structure and characteristics of the anode and cathode. In order that a molten carbonate fuel cell has stable output characteristics over a long period of time, the anode and cathode must satisfy the following requirements: the anode and cathode are chemically stable against a molten carbonate at a temperature of 600.degree. to 700.degree. C., have a capacity of guiding the fuel gas and oxidant gas to the reaction site through internal pores, are capable of sustaining guide paths of gases over a long period of time, of guiding the molten carbonate to the reaction site, of stably sustaining the guide path of the molten carbonate, have good electron conductivity, can reduce the contact resistance with the electrolyte layer or the interconnector, and have a large specific surface area so as to have a large number of three-phase surfaces (reaction points) where the electrodes, the reactive material, and carbonate ions are all present.
In view of these requirements, in a conventional molten carbonate fuel cell, a cathode and an anode comprise porous plates, which are obtained by sintering a nickel-based alloy powder to have a narrow pore size distribution.
When a cathode and anode of a material having a large average pore size, for example, 20 .mu.m or more, are used to stably supply the fuel and oxidant gases, the pressure of the supplied gas is applied to the electrolyte layer. This may reduce the number of reaction points. Then, output density cannot be improved. On the other hand, when a cathode and anode of a porous material having a small average pore size, for example, 2 .mu.m or less, are used to provide a large number of reaction points, the molten carbonate is guided toward the gas guide grooves by capillary action through the electrodes. Then, in this case, the electrolyte cannot be stably sustained. Furthermore, in this case, the electrolyte which has permeated into the pores results in an increase in the diffusion resistance of the gas, so that supply of reactive gas is prevented.
In order to solve these problems, an electrode has been proposed which comprises a sintered porous plate of a powdery material, wherein the average pore size is small toward the electrolyte layer and is large toward the gas supply side. When a cathode and anode of such a structure are assembled, pores having different sizes in the direction of thickness serve well to improve the output density and to sustain the electrolyte over a long period of time.
However, when a powdery material is sintered to form a porous electrode, the electrode has a porous structure wherein the points of powder particles contact each other. For this reason, when the porosity is set to be 50% or more, the electron conductivity is decreased and a voltage drop between the cathode or anode is increased. In addition, a porous electrode of such a structure has low mechanical strength and elasticity. The electrode may be broken due to stress applied by a shrinkage difference between the electrode and electrolyte layer when the cell temperature is decreased. The contact between the electrode and the interconnector may be made non-uniform to increase the contact resistance at this portion.